The present invention relates to a shadow mask of a color picture tube and a method of manufacturing the same.
In general, a color picture tube has an electron gun for generating three electron beams; a phosphor screen which is formed on the inner surface of a faceplate of an envelope to oppose the electron gun and which has red, blue and green emitting phosphors aligned in a predetermined sequence; and a shadow mask which opposes the phosphor screen at a predetermined distance (to be referred to as a q value hereinafter) therefrom and which has regularly formed apertures. In the color picture tube of this type, the three electron beams are converged in the vicinity of the apertures of the shadow mask and diverge in a space having the q value. The beams land on the corresponding phosphors to reproduce a color image.
This shadow mask is generally manufactured by the following process. A photosensitive layer is formed to a high-purity iron plate having a thickness of 0.1 to 0.3 mm. A mask pattern having a number of aperture images is lapped on the photosensitive layer. The photosensitive layer is exposed to the image of the mask pattern by a photo-exposure method. After development, drying and burning, the iron plate is etched, so that the iron plate has a number of apertures. The iron plate is pressed so that the portion of the iron plate which has the apertures is arcuated and that a peripheral portion thereof is formed to have a shape to be suitably mounted on a mask frame. The resultant structure is subjected to oxidation such that a dark gray or black oxide film having resistance to corrosion is formed on the surface, thereby obtaining a shadow mask. This oxide film serves: to prevent reflection of an ultraviolet ray on the shadow mask surface at the time when the phosphor screen is formed by the photo-exposure method through the shadow mask in the subsequent process; to prevent rusting before the picture tube is evacuated; to prevent the generation of secondary electrons; and to absorb the electron beam when the picture tube is operated. Oxidation methods such as steam oxidation, gas oxidation, or alkali bath oxidation can be used. The color of the oxide film is dark gray or black. In general, a blackish color is preferred.
The thickness of the oxide film preferably falls within the range between 1 .mu.m and 3 .mu.m, as described in Japanese Patent Disclosure No. 54-139463. When the thickness of the oxide film is less than 1 .mu.m, rusting cannot be completely prevented. On the other hand, when the thickness is greater than 3 .mu.m, splashing frequently occurs when the shadow mask is mounted in the color picture tube.
The material of the shadow mask generally comprises of a high-purity soft iron material. This material is selected in consideration of the supply capacity, cost, workability and strength. However, the major disadvantage of this material is its high thermal expansion coefficient of about 12.times.10.sup.-6 /.degree.C. in the temperature range of 0.degree. to 100.degree. C. An electron beam transmittance of the conventional shadow mask is about 15% to 25%. The remaining 75% to 85% of the electron beams bombard against the shadow mask, so that its kinetic energy is converted to thermal energy. As a result, the shadow mask is often heated to a temperature of 80.degree. C., and is subjected to a doming effect due to a high thermal expansion coefficient. Therefore, the q value locally deviates from the rated value. Such a change in the q value causes mislanding of each electron beam with respect to a corresponding phosphor, thereby degrading color purity. This tendency conspicuously occurs in a thin shadow mask having a fine aperture pitch for a high-resolution color picture tube. This problem becomes a decisive factor in the overall quality of the color picture tube.
In order to prevent the degradation of color purity, an alloy which contains as major constitutents iron and nickel and which have a thermal expansion coefficient of 5.times.10.sup.-6 /.degree.C. or less (1/10 the thermal expansion coefficient of iron) in the temperature ramge of 0.degree. to 100.degree. C. is used as a material of the shadow mask, as described in Japanese Patent Publication No. 42-25446, Japanese Patent Disclosure No. 50-58977 and Japanese Patent Disclosure No. 50-68650. In other words, a material having a low thermal expansion coefficient is used to substantially solve the doming effect.
However, since a material containing as the major constituents iron and nickel also tends to rust like soft iron during the manufacturing process, the apertures may clog and the withstand voltage characteristics of the shadow mask may be degraded. In order to prevent this, an oxide film is formed on the surface of the shadow mask. However, it is very difficult to form a black oxide film with high heat-resistive characteristics and good adhesion on the above-mentioned alloy material. A satisfactory oxide film cannot be formed on the alloy plate surface under the normal conditions of a steam atmosphere at a temperature of 570.degree. to 600.degree. C., or a CO+CO.sub.2 +O.sub.2 gas atmosphere at a temperature of 570.degree. to 600.degree. C. Even if the oxidation time is greatly prolonged (60 to 90 minutes as compared with the normal oxidation time of 5 to 10 minutes) to form an oxide film to a thickness of 1 to 3 .mu.m, adhesion between the oxide film and the iron-nickel alloy plate is weak. The oxide film tends to peel off the plate and forms dust within the picture tube, thereby degrading the voltage withstanding characteristics.
The above problem is assumed to be caused by the following phenomenon. In general, an iron shadow mask is oxidized in a steam atmosphere or a CO+CO.sub.2 +O.sub.2 gas atmosphere of 570.degree. to 600.degree. C. for 5 to 10 minutes to form an oxide film. The resultant oxide film is confirmed to comprise Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. The Fe.sub.2 O.sub.3 +Fe.sub.3 O.sub.4 oxide film is firmly formed on the underlying iron plate and will not peel off even after the resultant structure is heated. In this manner, the oxide film can serve to prevent the shadow mask from being corroded. On the other hand, in the shadow mask having as major constituents iron and nickel, a satisfactory oxide film cannot be obtained even if the same oxidation conditions as in the case of the iron shadow mask are given. In order to obtain a sufficient thickness of the oxide film formed on the iron-nickel alloy plate, the oxidation time is increased to obtain a desired thickness. However, in this case, cracks occur in the oxide film during the subsequent heat treatment, and the oxide film peels off the iron-nickel plate. When the oxide film was analyzed in order to inquire into the causes of these phenomena, it was found that the oxide film contained nickel oxide besides Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. As a result, it is presently assumed that an oxide film having a sufficient thickness cannot be formed on the iron-nickel shadow mask since the iron concentration is low at the surface region of the plate; and that the oxide film can peel off from the plate during heat treatment since the thermal expansion coefficients of the oxide film and the plate greatly differ from each other.